Fossils Discovery Kit

Common Types of Body Fossil Preservation.

Proetidae trilobite.

The aim of this activity is to examine the different types of fossil preservation most commonly found in the rock record and begin to understand how the fossil record can be biased in the type of organisms most likely to be preserved.

1) Unaltered – Some organisms produce really hard parts or live in habitats with just the right conditions that their skeletal remains are unchanged or unaltered from when the organism was alive. Examples of this can include: vertebrate teeth, invertebrate shells, or organisms preserved in amber or mummified. 

This blastoid, an extinct type of echinoderm, had a shell composed of calcium carbonate is hard enough that the fossil was unaltered.

 2) Permineralization – The hard parts of an organism are usually part of its skeleton, and most skeletons have some percentage of soft tissue incorporated into their structure. Take human’s bones, for example; they are not solid structures, but have numerous small and large spaces in them that help provide strength and are filled with blood vessels, nerves, fat, and other specialized cells. These soft parts decay shortly after death, leaving open spaces in the bone. If these spaces become filled in with minerals, it is called permineralization. Most bone and wood is permineralized. Calcite (calcium carbonate) and silica (quartz) are two of the most common minerals in permineralization. 

3) Compression – If the fossil remains have been flattened, so the original three-dimensional shape of an organism has been lost, this is called compression. Sometimes, the weight of the overlying rock or sediment covering the organism can compress it so much that only a thin film or an impression of the organism is left. This type of preservation is common with plants, fish and sometimes with organisms that are entirely composed of soft-tissue, like jellyfish, squids, or worms. Plants are commonly found in the fossil record, because their cell walls are composed of cellulose and lignin, which are as strong as bone. A lot of the seeds and pollen produced by plants are many times stronger than your teeth and are produced in great abundances, making them more likely to end up in the fossil record.

4) Replacement – This type of fossil preservation occurs when the original mineral that made up the hard parts of an organism is replaced by an entirely different mineral. The replacement process typically happens very slowly over a long time, so the fine details of a fossil are preserved. Common replacement minerals include: pyrite (fool’s gold), silica (quartz), iron, opal, or phosphate. 

5) Recrystallization – Sometimes after an organism dies and it is buried, the tiny crystals in the mineralized portion of an organism’s hard parts will begin growing again. This is called recrystallization. Fine details in the fossil are usually lost, because the crystals are growing larger and larger, destroying a lot of the structure. Any open spaces in the fossil are usually filled with interlocking crystals. Recrystallization is most common in invertebrates. The most common minerals that make up the shells of invertebrates are calcite or aragonite. Calcite is more common than aragonite and will be off-white to gray in color. Aragonite can be recognized by its rainbow-like colors. Calcite and aragonite can both become a chalky white when they become weathered. 

6) Mold – A very long time after an organism dies and is buried, the soil surrounding the organism will turn into rock. This process if called lithification and takes over hundreds of thousands of years.  The remains of the organism are now entombed in a hard rock and can be dissolved away, leaving behind an empty space that is in the organism’s shape. This is called a mold. Think of a mold as a three dimensional impression of an organism. 

7) Cast – To create a cast fossil, first a mold has to form (see #6). If this empty space of a mold becomes filled in with sediment or a mineral, it creates a three-dimensional copy or cast of the organism. Molds and casts are most common with shelled invertebrates. 


Test your knowledge of fossil preservation with this activity! For each fossil shown below, think critically to predict which type of fossil preservation has occurred. Remember, your options are:

  1. Unaltered
  2. Permineralization
  3. Compression
  4. Replacement
  5. Recrystallization
  6. Mold
  7. Cast
Fossil Fishes - These fossil fish are Eocene (56 to 33.9 million years old) in age. These fossils came from a famous package of rock called the Green River Formation, which was deposited by a large lake system found in Wyoming. Colorado and Utah during the Eocene. Fish are part of a group of vertebrates called Osteichthyes or bony fish. Which type of fossil preservation do you think has occurred? Why do you think that?
Mollusk: Bivalves - These are bivalves. Their fossil record spans from the early Cambrian (~540 million years ago) to today. Most live in saltwater environments, but there are some freshwater species. Which type of fossil preservation do you think has occurred? Why do you think that?
Hexagonaria Coral - This fossil coral is part of the genus Hexagonaria. Hexagonaria lived during the Devonian (419 to 358.9 million years ago) and is commonly found in Michigan. The majority of corals produce a hard, external skeleton. They are major reef builders and are only found in saltwater environments. Most corals are colonial organisms, meaning that a single, living, functional organism is composed of multiple individuals living closely together, sharing a digestive tract. 'Hexa' means six and refers to the six-sided compartments that each individual lives in, to make up one living, functional coral. Which type of fossil preservation do you think has occurred? Why do you think that?
Ammonite - This fossil ammonite is part of the genus Cleoniceras. Ammonites are part of a group called cephalopods, which includes octopus and squids. Ammonites are mollusks that look like a squid with a shell. They are more closely related to a living octopus, though the shells resemble that of living nautiloids. Which type of fossil preservation do you think has occurred? Why do you think that?
Brachiopod- This fossil is a brachiopod, part of the genus Platystrophia. It is an invertebrate with a shell consisting of two halves. Platystrophia fossil record spans from the middle Ordovician to the middle Silurian era (470 to 427 million years ago). Which type of fossil preservation do you think has occurred? Why do you think that?
Shark Tooth - This tooth is from the genus Carcharocles. Their fossil record spans from the early Eocene (50 million years ago) to the end of the Pleistocene (11,700 years ago). Sharks are usually represented in the fossil record by their teeth because they have a skeleton entirely made of cartilage, the same flexible material that makes up your nose and ears. Most sharks continuously replace their teeth throughout their life. A shark can lose between 20,000 to 30,000 teeth during its lifetime! Which type of fossil preservation do you think has occurred? Why do you think that?